Conventionally, the video camera enhances edges of a picture to produce a video signal for an object having well-defined edges and an improved sharpness.
The edge enhancement circuit conventionally used in the video cameras will be described below with reference to FIG. 1. The edge enhancement circuit is generally indicated with a reference 101.
As shown in FIG. 1, the edge enhancement circuit 101 includes a bandpass filter (BPF) 111, gain control circuit 112, amplitude limiting circuit 113 and an addition circuit 114.
The edge enhancement circuit 101 is supplied with signals of a picture picked up.
The video signal supplied to the edge enhancement circuit 101 are supplied to the BPF 111 where a high frequency component will be extracted from the video signal. The high frequency component of the video signal contains an edge component of the picked-up picture. That is, the BPF 111 will provide the edge component representing the edge of the picture. The edge component output from the BPF 111 is supplied to the gain control circuit 112 where the extracted edge component will be multiplied by a predetermined gain adjustment coefficient to adjust the extent of the edge enhancement. The output from the gain control circuit 112 is supplied to the amplitude limiting circuit 113. The amplitude limiting circuit 113 limits the signal level of a part of the output from the gain control circuit 112, higher than a predetermined signal level, and provides a final detail signal (edge enhancement signal). The detail signal is supplied to the addition circuit 114. The addition circuit 114 generates a detail signal and adds it to the original main video signal.
The video signal having the detail signal added thereto as above will provide a picture having the edge thereof enhanced and thus showing the object having a well-defined edge.
Incidentally, the user of the video camera, namely, the photographer, wants to change the coefficients for edge enhancement according to his taste or a scene to pick up in some cases. In such cases, he or she should change the frequency (thickness) and signal level of the detail signal to be added to the original video signal.
For a picture containing more high frequency components of a spatial frequency of a video signal such as picture taken with the lens zoomed to the wide angle side, for example, the frequency characteristic of detail signal is set for emphasis of the high frequencies. Thus, the edge will be emphasized to be thin, resulting in a natural picture having the edge thereof emphasized with a higher definition. Also, for a picture containing more low spatial-frequency components of a video signal such as a picture taken with the lens zoomed to the telephoto side, the frequency characteristic of the detail signal is set for emphasis of the low frequencies. Thus, the edge will be emphasized to be thick, resulting in a well-defined picture with an improved sharpness.
In a video camera which does not assure any high naturalness and good S/N ratio of a picture, such as an ordinary household type, to emphasize the edge of a picture whose spatial frequency is high, the signal level of the detail signal should be set lower to prevent the S/F ratio from being worse in order to provide a clearer picture, while to emphasize the edge of a picture whose spatial frequency is low, the signal level of the detail signal should be set higher to provide a preferable picture whose sharpness is higher. On the contrary, however, in a video camera which assures a high definition and good S/N ratio of a picture, such as a broadcasting type, to emphasize the edge of a picture whose spatial frequency is higher, the signal level of the detail signal should be set higher to improve the sharpness, while to emphasize the edge of a picture whose spatial frequency is lower, the signal level of the detail signal should be set lower to provide a desirable picture whose definition is higher without being excessively emphasized.
To change the frequency characteristic for the edge enhancement as above, it is generally sufficient to change the frequency (thickness)of an edge component to emphasize by changing the passband frequency characteristic of the BPF 111 in the edge enhancement circuit 101. Generally in a video camera adopting the digital signal processing, the passband frequency characteristic of the bandpass filter can be changed by a built-in CPU (central processing unit). For example, the coefficient of the bandpass filter is variable, and the passband frequency characteristic can be changed by alerting the coefficient. Also, there have been proposed some video cameras in which there is provided, for example, a plurality of fixed-coefficient filters different in frequency character is provided in such a manner that signal outputs from the filters can be added together and the frequency characteristic can be changed by altering the ratio of mixing the output signals.
It should be reminded that since a picture being picked up always varies, the spatial frequency of the picture will not always be constant. Therefore, the spatial frequency of the picture changes in the course of a shooting, with the result that no optimum edge enhancement is possible with a set passband frequency characteristic and the frequency characteristic will be worse in some cases.
For example, if a picture being picked up with the lens originally set to the wide angle side and with settings for enhancement of the high frequency component of the frequency characteristic of a detail signal is zoomed in with the lens set to the telephoto side, it will blur without any sufficient edge enhancement irrespectively of the settings for the enhancement of the high frequency component since the spatial frequency of the video signal contains many low-frequency components. On the contrary, however, if a picture being picked up with the lens originally set to the telephoto side and settings for enhancement of the low-frequency component of the frequency characteristic of the detail signal is zoomed out to the wide angle side, it will have the high frequency component thereof insufficiently edge-enhanced irrespectively of the settings for the enhancement of the low frequencies of the frequency characteristic of the detail signal since the spatial frequency of the video signal contains more high-frequency components.
For generating a detail signal, however, a filter coefficient etc. are set before a shooting, they can hardly be changed during the shooting. Therefore, the conventional video camera cannot provide any appropriate edge enhancement in case the spatial frequency characteristic of a video signal during a shooting has changed.
To accommodate the variation of the spatial frequency due to the zoom-out and -in during a shooting, however, it is also possible to change the frequency characteristic of a detail signal or the gain by feeding back information about the zoomed position of the lens to the CPU built in the video camera. However, zooming of the lens to the wide angle side will not result in that the spatial frequency of a picture contains solely high frequencies over the display of the picture. Thus, even if the frequency characteristic and gain are changed according to a zoomed position of the lens, an unnatural edge-enhanced part will take place in the picture. Also, a time lag will take place until zooming information about the lens is reflected by the control of the detail signal, and the time lay will result in an unnatural edge-enhancement as the case may be.